Why is it colder after a snowfall?

Dave from Lafayette asked “it is colder after snow than before. How much of that has to do with the albedo effect?”

Dave’s certainly right that it tends to be colder after good snowfall. His explanation, that the snow reflects incoming solar radiation, is partially correct, but there’s more to it.

In order to understand what’s happening when snow covers the ground, we must first understand how temperature changes. The air is not heated directly by incoming solar radiation. The wavelength is too short to allow for much of that. Instead, the radiation heats the planet’s surface. The warmed surface warms air in the very lowest layers of the atmosphere (i.e. less than a foot above ground) by contact, and also emits long-wave (infrared) radiation that heats the atmosphere.

Snow affects this process in two important ways. The first, as Dave suggested, is by reflecting 80-90% of incoming solar radiation. The second is by acting as a blanket, trapping the warmth in the soil. Snow crystals are mostly empty air, and as a result are very good insulators (which is why igloos can keep people warm). The trapped heat begins melting the snow from the bottom, but doesn’t have a chance to help warm the air.

There are other considerations as well. The large-scale weather conditions can have an effect, too. The passage of a cold front, which is often associated with heavier snows anyway, will obviously have a cooling effect. The amount of cloud cover will also have an effect. Clear nights with snow cover will be much colder than cloudier or less snowy nights. Unfortunately, I was not able to find a good source to answer the “how much” part of Dave’s question, but at least we know the factors that play a part.